1. Field of the Invention
The present invention relates to a film pressure sensitive resistor in which a resistance value varies according to a pressure, a load, and the like, and to a pressure sensitive sensor made therefrom.
2. Description of Related Art
As a conventional pressure sensitive resistor, for example, a pressure sensitive resistor having a structure as shown in FIGS. 1A to 1C is known. FIG. 1A is a cross sectional view showing an internal structure of a pressure sensitive resistor using three electrodes. FIG. 1B is a plan view showing an electrode pattern and a wiring pattern formed on an upper circuit board in the pressure sensitive sensor shown in FIG. 1A. FIG. 1C is a plan view showing an electrode pattern formed on a lower circuit board in the pressure sensitive sensor shown in FIG. 1A.
In the upper circuit board 1 according to this example, an electrode 2 shaped like the teeth of a comb and a wiring part 3 for connecting the electrode 2 and an external device (not shown) are formed, and furthermore, an electrode 4 shaped like the teeth of a comb and insulated from the electrode 2, and a wiring part 5 for connecting the electrode 4 and an external device (not shown) are formed as shown in FIG. 1B. The electrode 4 is formed on the same surface on which the above electrode 2 is formed. Furthermore, an electrode 7 shaped like a disc is formed on a lower circuit board 6 as shown in FIG. 1C. A spacer sheet 8 is provided between the upper circuit board 1 and the lower circuit board 6, and the upper circuit board 1 and the lower circuit board 6 are fixed by adhesion layers 9 which are provided between the upper circuit board 1 and the spacer sheet 8, and between the lower circuit board 6 and the spacer sheet 8. Furthermore, a space corresponding to the thickness of the spacer sheet 8 is formed between the electrodes 2 and 4 on the upper circuit board 1 and the electrode 7 on the lower circuit board 6.
In order to operate the above layer structure as a pressure sensitive sensor, at least one selected from the above three electrodes 2, 4, and 7, and at least a part of the electrode selected, must be formed as a film pressure sensitive resistor.
In the pressure sensitive sensor comprising the above compositions, when the pressure sensitive sensor has a load applied thereto from the upper circuit board 1 side, a part of the upper circuit board 1 is deformed and the electrodes 2 and 4 of the upper circuit board 1 contact the electrode 7 of the lower circuit board 6, and as a result, the electrodes 2 and 4 conduct to the lower circuit board 6 through the electrode 7. Furthermore, when the pressure sensitive sensor has a larger load applied thereto, the contact area of the electrodes 2 and 4 to the electrode 7 increases, and as a result, conductive resistance is decreased.
As the other pressure sensitive sensor, for example, a pressure sensitive sensor having a structure shown in FIGS. 2A to 2C is known. FIG. 2A is a cross sectional view showing an internal structure of the pressure sensitive sensor using two electrodes which are oppositely provided. FIG. 2B is a plan view showing an electrode pattern and a wiring pattern formed on an upper circuit board in the pressure sensitive sensor shown in FIG. 2A. FIG. 2C is a plan view showing an electrode pattern formed on a lower circuit board in the pressure sensitive sensor shown in FIG. 2A.
In this example, the electrode 7 shaped like a disc is formed on the lower circuit board 6 and a wiring part 10 of an external device (not shown) is connected with the electrode 7. On the other hand, an electrode 11 shaped like a disc is formed on the upper circuit board 1 so as to be opposite to the electrode 7 on the lower circuit board 6. The electrode 11 is connected with a wiring part 12 of an external device (not shown).
In this type of pressure sensitive sensor, when the pressure sensitive sensor has a load applied thereon from the upper circuit board 1 side, a part of the upper circuit board 1 is deformed and the electrode 11 of the upper circuit board 1 contacts the electrode 7 of the lower circuit board 6, and as a result, the electrodes 7 and 11 conduct. Furthermore, when the pressure sensitive sensor has a larger load applied thereto, the contact area of the electrodes 7 and 11 increases, and as a result, conductive resistance is decreased.
Since variation in conductive resistance is caused by variation in contact resistance of the film pressure sensitive resistor and the opposite electrodes (Japanese Examined Patent Application, Second Publication, No. Hei 2-49029), it is expected that the properties of the above pressure sensitive sensor comprises a property such that the conductive resistance gradually decreases according to a gradual increase the load on the pressure sensitive sensor, that is, the conductive resistance gently changes depending on changes in the load on the sensor according to the curve shown in FIG. 3.
However, in the conventional pressure sensitive sensor used in practice, since the resistance due to load varies steeply as the curve a shown in FIG. 4, the pressure sensitive sensor functions merely as a switch. Therefore, it is difficult for a conventional pressure sensitive sensor to detect a load in the low load range when contact of a upper electrode and a lower electrode starts, and as a result, the measurable load range is not wide.
Furthermore, in the conventional pressure sensitive sensor, since the resistance-load curve is not smooth and has steps as in curve b shown in FIG. 4, the step parts cannot be measured.
Furthermore, in conventional pressure sensitive sensors, since there is variation in performance among sensor products, the sensors lack operational reliability. Additionally, conventional pressure sensitive sensors have insufficient environmental resistance and durability to repeated use (depression).
The present invention was made in order to solve the above problems, and an object of the present invention is to provide a film pressure sensitive resistor and a pressure sensitive sensor using the resistor in which the measurable load range is wide, the resistance-load characteristics show a smooth curve, their qualities are stable, and superior environmental resistance and durability to repeated use (depression) are obtained.
In order to solve the above problems, in the first aspect of the present invention, there is provided a film pressure sensitive resistor having an average surface roughness of 0.1 xcexcm to 3 xcexcm, a peak-to-peak concavo-convex period of 10 xcexcm to 1000 xcexcm, and an elastic modulus of 800 MPa to 8000 MPa.
As described above, since the above film pressure sensitive resistor comprises sufficient average surface roughness, sufficient peak-to-peak concavo-convex period, and sufficient elastic modulus, the contact area gently changes according to changes in load in the low load range when contact is started.
The above film pressure sensitive resistor may comprise conductive particles, a binder, and spherical elastic particles having an average particle diameter of 2 xcexcm to 50 xcexcm.
According to the above film pressure sensitive resistor, appropriate resistivity in the film is obtained and changes in resistance according to changes in load are easily exhibited.
In the above film pressure sensitive resistor, the elastic particles may be organic elastic fillers.
According to the above film pressure sensitive resistor, surface roughness, peak-to-peak concavo-convex period, and elastic modulus which are required by the film pressure sensitive resistor are obtained.
In the above film pressure sensitive resistor, the elastic particles may have an average particle diameter of 13 xcexcm to 20 xcexcm.
According to the above film pressure sensitive resistor, appropriate resistivity of the film is obtained and changes in resistance according to changes in load are easily exhibited.
In the above film pressure sensitive resistor, the conductive particles may be graphite or carbon black.
According to the above film pressure sensitive resistor, appropriate resistivity of film is obtained and changes in resistance according to changes in load are easily exhibited.
In the above film pressure sensitive resistor, at least a part of the binder may comprise a polyester resin.
According to this film pressure sensitive resistor, adhesiveness to a circuit board is improved.
In the elastic particles having an average particle diameter of 13 xcexcm to 20 xcexcm, the elastic particles are 10% to 16% by volume of the film pressure sensitive resistor.
According to this film pressure sensitive resistor, the measurable range of resistance in the low load range becomes wide.
Furthermore, in a second aspect of the present invention, there is provided a pressure sensitive sensor comprising at least two electrodes which are provided with a space therebetween when no load is applied and contact to conduct when a load is added, wherein conductive resistance is reduced according to the increase of the load, and at least a part of at least one of the electrodes is formed by the film pressure sensitive resistor according to any one of the above-mentioned film pressure sensitive resistors.
According to the pressure sensitive sensor, conductive resistance gently changes according to load changes in the low load range when contact is started, and the measurable range becomes wide. Furthermore, according to this pressure sensitive sensor, since the resistance-load curve does not have any steps, the entire measurable range is measured.
Furthermore, according to the pressure sensitive sensor of the present invention, individual differences between sensors are narrowed and excellent environmental resistance and durability to repeated use (depression) are obtained. Therefore, this pressure sensitive sensor is used for, for example, a sensor for detecting the weight of a person sitting on a chair, a sensor for input of instruments or electronic equipment, and the like.